The avian acute leukemia viruses show marked cell-type specificity, some viruses transforming cells of the erythroid series, other cells of the granulocyte/macrophage series. The objective of this work is to determine the basis of this target-cell specificity and to define the viral functions which affect the differentiation of the target cells. We will examine the infection of cultured erythroblasts and macrophages by the avian acute leukemia viruses, in order to determine if viral replication or gene expression are cell-type specific. In order to define the viral functions which cause the block in haemopoietic cell differentiation, new isolates of acute leukemia viruses from chickens infected by transformation-defective mutants of Rous sarcoma virus will be characterized. In addition we will attempt to isolate viral mutants in which the ability of the virus to block differentiation is temperature-sensitive.